Long Term Evolution (LTE) networks use Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) and Discrete Fourier Transform (DFT) spread OFDM in the uplink (UL). The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. Within the grid illustration, then, each column represents one OFDM symbol interval, each row represents one subcarrier of a defined subcarrier frequency, and each cell in the grid represents a given resource element or RE.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms. FIG. 2 illustrates the frame/subframe structure. Transmission and reception from a given node, e.g. a terminal in a cellular system such as LTE, can be multiplexed in the frequency domain or in the time domain (or combinations thereof). Frequency Division Duplex (FDD) implies that downlink and uplink transmission take place in different, sufficiently separated, frequency bands. Time Division Duplex (TDD) implies that downlink and uplink transmission take place in different, non-overlapping time slots.
TDD operation thus allows a single carrier frequency and uplink and downlink transmissions are separated in time. Because the same carrier frequency is used for uplink and downlink transmission in TDD, both the base station and the mobile terminals need to switch from transmission to reception and vice versa. An essential aspect of any TDD system is to provide the possibility for a sufficiently large guard time where neither downlink or uplink transmissions occur. This is required to avoid interference between uplink and downlink transmissions. For LTE, this guard time is provided by special subframes (subframe 1 and, in some cases, subframe 6), which are split into three parts: a downlink part (DwPTS), a guard period (GP), and an uplink part (UpPTS). FIG. 3 illustrates the special subframe structure.
According to that structure, a special subframe can be understood as being both an uplink and a downlink subframe, in that it has a portion used for the downlink and a portion used for the uplink. Regular or normal subframes, i.e., the subframes that are not “special” according to the foregoing definition, are either allocated to uplink or downlink transmission.
TDD allows for different asymmetries in terms of the amount of resources allocated for uplink and downlink transmission, respectively, by means of different downlink/uplink configurations. That is, different cells within an LTE or other type of cellular communication network may use different Uplink/Downlink (UL/DL) configurations, meaning that different cells have different allocations of uplink subframes and downlink subframes.
In the LTE example, there are seven different configurations (“Configuration 0” through “Configuration 6”) as shown in FIG. 4. Further, there are nine different configurations for special subframes (“Configuration 0” through “Configuration 8”), as shown in FIG. 5. In FIG. 5, cross-hatched blocks represent DL OFDM symbols, diagonally-hatched blocks represent UL OFDM symbols, and empty blocks represent guard time.
Turning to another concept momentarily, the communication specifications referred to as “Release 8,” as promulgated by the Third Generation Partnership Project (3GPP), supported bandwidths up to 20 MHz. To meet the IMT-Advanced requirements, however, Release 10 supported larger bandwidths. For compatibility, a Release 10 LTE carrier wider than 20 MHz appears to a Release 8 wireless device or terminal as more than one LTE carrier, with each such carrier referred to as a “component carrier” having a bandwidth of 20 MHz or less. For efficiency, Release 10 made it possible to have a wideband carrier that allowed for legacy Release 8 terminals to be scheduled within all parts of the wideband carrier. Carrier Aggregation or “CA” provides for such efficiency.
Carrier Aggregation implies that an LTE Release 10 (Rel-10) terminal can receive multiple component carriers, where the component carriers have, or at least the possibility to have, the same structure as a Rel-8 carrier. As a general proposition, interference in the network is reduced or at least more easily managed when the neighboring cells associated with an aggregated set of component carriers all use the same UL/DL configuration—i.e., the same allocations of uplink and downlink subframes within a frame.
CA as implemented in Rel-10 was based on having the same UL/DL configuration among all cells configured as serving cells in a CA configuration. Rel-11 changes that, however, and allows the aggregation of component carriers having different UL/DL configurations. The change provides more flexible aggregation scenarios, such as aggregating carriers within a heterogeneous network that provides radio coverage over a certain geographic area using a mix of high-power base stations and low-power base stations or access points.
The ability to aggregate carriers having different UL/DL configurations also provides more flexibility in adapting to changing traffic patterns—i.e., to change the uplink-to-downlink subframe allocations within a given cell or cells of the network, to reflect the actual traffic patterns of terminals operating within those cells.
With the introduction of different UL/DL configurations on different carriers, two types of TDD terminals must be considered. A first type is referred to as being full duplex, meaning that it can simultaneously transmit on one carrier while receiving on another carrier. A second type of TDD terminal is referred to a being half duplex, because it cannot transmit and receive at the same time.
It is recognized herein that a number of complex challenges arise in the context of a half-duplex TDD terminal that has CA serving cells with differing UL/DL configurations. FIG. 6 illustrates one aspect of the challenges. Consider a CA configuration involving a Primary Cell (PCell) having one UL/DL configuration and a Secondary Cell (SCell) having a different UL/DL configuration. Because the two cells use different UL/DL configurations, special subframes in the PCell, which the terminal is obligated to process, may coincide with normal downlink subframes in the SCell. FIG. 6 illustrates a special subframe 10 in the PCell coinciding or overlapping with a normal downlink subframe 12 in the SCell. “Coinciding” in this sense means at least partly overlapping in time—e.g., assuming synchronized frame timing between the primary and secondary cells, the normal downlink subframe 12 in the secondary cell is transmitted at the same time as the special subframe in the primary cell.
FIGS. 7 and 8 illustrate the logical structure of an LTE subframe with respect to certain control channels. FIG. 7 in particular shows a control region that comprises the first four OFDM symbols of the subframe. Physical Downlink Control Channel (PDCCH) transmissions occur within this control region. The OFDM time-frequency grid within the control channel region offers a resource element or RE, which represents the time-frequency intersection of one subcarrier within one OFDM symbol time. Resource Element Groups or REGs are built up from four REs, and Control Channel Elements or CCEs aggregate nine REGs. In turn, a PDCCH aggregates a number of CCEs, with the number of CCEs aggregated for a given PDCCH being referred to as its CCE aggregation level. PDCCHs are used to transmit Downlink Control Information (DCI) to targeted terminals.
In contrast, FIG. 8 illustrates that the enhanced PDCCH (ePDCCH) is transmitted across the latter portion of the subframe in so called enhanced control regions. Consequently, a terminal is not expected to receive ePDCCH in a special subframe with special subframe configuration 0 or 5, where a normal Cyclic Prefix (CP) is used. Nor is the terminal expected to receive the ePDCCH in a special subframe having configuration 0, 4, or 7 in extended CP. Similarly, while the Physical Downlink Shared Channel (PDSCH) is not illustrated, it extends over the data portion of a subframe and a terminal targeted by the transmission is not expected to receive the PDSCH in a special subframe with special subframe configuration 0 or 5 in normal CP, or in a special subframe configuration 0 or 4 in extended CP. Still further, the abbreviation of the downlink portion within special subframes also means that a terminal is not expected to receive Demodulation Reference Symbol (DMRS) transmissions in a special subframe.